Recently, along with an increase in amount of data transferred on a network, communication networks are demanded to have a transfer capacity equal to or more than 10 gigabits per second (10 G (b/s)). To achieve such a large capacity transfer, an electro absorption (EA) modulator or the like capable of modulating light at high speed have been used.
However, in the EA modulator, a characteristic of the magnitude of voltage applied to the EA modulator and the intensity of signal light output from the EA modulator (hereinafter, “extinction characteristic”) is fluctuated depending on a temperature, and thus there is a necessity of maintaining the temperature constant to suppress fluctuation of the extinction characteristic. In addition, there is also a problem that the extinction characteristic is fluctuated due to degradation of the EA modulator with age or the like, and thus the waveform of the signal light output from the EA modulator is degraded.
Accordingly, there has been proposed a method in which a low frequency pilot signal is superposed on a main signal (transfer signal) to input it to an EA modulator, a pilot signal component is extracted from signal light output from the EA modulator, and based on the extracted pilot signal, voltage to be applied to the EA modulator is controlled, thereby suppressing the degradation of the waveform of the signal light due to fluctuation of the extinction characteristic (for example, see Japanese Laid-open Patent Publication No. 10-123471 and Japanese Patent No. 3333133).
However, generally, an operating point of a signal source circuit of a main signal and that of an EA modulator (bias voltage) do not always match (or it is difficult to match these points). Therefore, AC coupling is often adopted between the signal source of the main signal and the EA modulator.
When the signal source of the main signal and the EA modulator are AC-coupled, a low-range cutoff frequency is limited by a coupling condenser. Thus, when the conventional method in which the pilot signal is superposed on the main signal is used, it is necessary to lower the low-range cutoff frequency by using a large capacity condenser in order to flow not only the main signal but also the pilot signal without causing a distortion.
When the capacity of the condenser is increased to lower the low-range cutoff frequency, the size of the condenser becomes large, and consequently the size of an electrode are made larger. This makes it difficult to design a characteristic impedance of a substrate pattern. Further, as to the condenser, its impedance at a high frequency becomes higher as the size increases, and thus it is disadvantageous for the high frequency characteristic.
Accordingly, it has been a very important issue to suppress fluctuation of a waveform of signal light output from an EA modulator without using a large capacity condenser even when an extinction characteristic of the EA modulator is fluctuated.